CD137-expressing human NK cells lyse NK-sensitive tumor targets less efficiently than non–CD137-expressing human NK cells. (A) IL-2–stimulated NK cells were cultured for 24 hours in the presence of immobilized GG mAb (●) or immobilized GA mAb (■) and subsequently used in a 4-hour 51Cr-release assay to determine lytic activity against K562 target cells. Non–CD137-expressing NK cells (■) and CD137 expressing NK cells (●) were added at the indicated effector-to-target ratios (E/T). Data represent mean and standard deviation of 2 independent experiments. Lytic activity of CD137-expressing NK cells (i-GG cultured) was significantly diminished compared with non–CD137-expressing NK cells (i-GA cultured) with *P = .005. Error bars represent SD. (B) Expression of degranulation marker CD107a on IL-2–stimulated NK cells after 6 hours of culture with i-GG. Gates were set around live cells based on forward and site scatter plots and numbers in the dot plots indicate the percentage of NK cells expressing CD107a. Experiment shown represents 1 of 4 individual experiments. Statistical analysis is based on 4 experiments performed with P = .011 for i-GG compared with i-GA. Immobilized mAb culture conditions are indicated with i-. GG indicates glycosylated chimeric anti-CD137 mAb; GA, aglycosylated chimeric anti-CD137 mAb.
